The U.S. government is stepping up efforts to protect the planet from incoming asteroids that could wipe out entire regions or even continents.

The National Science and Technology Council released a report Wednesday calling for improved asteroid detection, tracking and deflection. NASA is participating, along with federal emergency, military, White House and other officials.

For now, scientists know of no asteroids or comets heading our way. But one could sneak up on us, and that's why the government wants a better plan.

NASA's planetary defense officer, Lindley Johnson, said scientists have found 95 percent of all these near-Earth objects measuring one kilometer (two-thirds of a mile) or bigger. But the hunt is still on for the remaining 5 percent and smaller rocks that could still inflict big damage.

Altogether, NASA has cataloged 18,310 objects of all sizes. Slightly more than 800 are 460 feet (140 meters) or bigger.

There's no quick solution if a space rock is suddenly days, weeks or even months from striking, according to Johnson. But such short notice would give the world time, at least, to evacuate the area it might hit, he said.

Ground telescopes are good at picking up asteroids zooming into the inner solar system and approaching from the night side of Earth, Johnson said. What's difficult to detect are rocks that have already zipped past the sun and are heading out of the solar system, approaching from the day side. That's apparently what happened in 2013 when an asteroid about 66 feet (20 meters) in size suddenly appeared and exploded over Chelyabinsk, Russia, damaging thousands of buildings and causing widespread injuries.

An asteroid double or even triple in size exploded over Tunguska, Russia, in 1908, leveling 770 square miles (2,000 square kilometers) of forest. According to the report released Wednesday, casualties could be in the millions if a similar event struck New York City.

A giant space rock wiped out the dinosaurs when it smacked into Mexico's Yucatan peninsula some 65 million years ago.

Johnson stressed it would take years to attempt to turn away a potential killer asteroid - several years to build a spacecraft then another few years to get it to the target. Ideally, he'd like at least 10 years' advance notice.

A mission to defend planet Earth could involve hitting the asteroid or comet with big, fast-moving robotic spacecraft in hopes of changing its path; or worst case, launching a nuclear device not to blow up the asteroid but rather to superheat its surface and blow off enough material to divert it.

All that involves current technology, Johnson said.

"Part of what this action plan is about is to investigate other technologies, techniques for both deflection and disruption of the asteroid," he told reporters.

Most of the extra work can be done with existing funds, said Aaron Miles of the White House Office of Science and Technology Policy.

"This is more about figuring out how to use those resources smartly," he said.

The bottom line, officials said, is the U.S. government wants to be prepared to decide which action is best if needed.

Scientists hope to learn a lot more about asteroids from a pair of missions currently underway. NASA's Osiris-Rex spacecraft will reach the asteroid Bennu later this year and return samples in 2023, and Japan's Hyabusa 2 is closing in on the asteroid Ryugu, with samples to be returned in 2020.

Forget about sending astronauts, Hollywood style.

"It makes a good movie, but we did not see in our study any technique that would require the involvement of astronauts," Johnson told reporters. Missions like this lasting months or years make it difficult if not impossible for humans, given current technology, he noted.